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Starting from MathSciNet, click on the Citations tab.
Narrow subject searches by citation find most-cited publications in a specialized subfield.
Broad subject searches by citation find the most-cited expository books.
Example: What is the most cited book in probability theory?
Look up the MSC for Probability Theory. It is "60."
Starting from MathSciNet, click on "Citations" tab.
Click on "Search by Subject" tab.
Input "60" in "Subject classification" search box.
Answer is Convergence of Probability Measures, by Patrick Billingsley. The reviewer, M. M. Siddiqui, describes the book as "welcome," "lucid," and "elegant." Scroll down to find that the second edition has a separate listing, with yet more citations.
Print copy of the 1968 first edition is on the 4th floor of LSU Library, at QA273. B574 (Catalog record)
Print copy of the 1999 second edition is nearby at QA273.6. B55 (Catalog record)
See here for more about MathSciNet as a citation database.